FIG. 6 shows a conventional light detector.
In FIG. 6, reference numerals 1 to 4 represent substantially square divisional light-receiving areas which are separated one from one another by separating areas 5, each having a uniform width.
Reference symbol A represents an actual light-receiving area to which a bundle of luminous flux (light spot) is applied. The area A is circularly shaped so as to equally and partially cover respective divisional light-receiving areas 1 to 4.
FIG. 7 is a circuit diagram for obtaining a tracking error signal from the output of the light detector shown in FIG. 6 by means of a push-pull system.
In FIG. 7, reference numeral 11 represents an adder for adding the respective outputs of divisional light-receiving areas 1 and 2; 12 represents an adder for adding the respective outputs of the divisional light-receiving areas 3 and 4; and 13 represents a subtracter for subtracting the output of adder 12 from the output of adder 11.
Next, the operation will be described.
In an optical disk, a bundle of luminous flux is received by several areas, that is, the four divisional light-receiving or light-detecting areas 1 to 4 in this example, and a signal for controlling the position of the bundle of luminous flux (tracking error signal) is obtained by comparing the strengths or phases of the respective outputs of the respective divisional light-receiving areas 1 to 4.
For example, in the case where a tracking error signal for tracking an information signal track is obtained by a push-pull system, and when the information signal track moves in the direction of arrow X in FIG. 6, that is, in the track-running direction while a disk is rotating, subtracter 13 subtracts the output of adder 12 for adding the respective outputs of divisional light-receiving areas 3 and 4 from the output of adder 11 for adding the respective outputs of divisional light-receiving areas 1 and 2, so that a tracking error signal (a differential signal) TE corresponding to the quantity of displacement in tracking can be obtained as shown in FIG. 8.
Tracking error signal TE is a signal corresponding to the quantity of tracking displacement in the direction of arrow Y in FIG. 6.
Accordingly, it is possible to perform proper tracking control by correcting the quantity of tracking displacement on the basis of the tracking error signal produced from subtracter 13.
In order to facilitate the adjustment of the light detector and to prevent the sensitivity of the light detector from being lowered even if the bundle of luminous flux is displaced on the light detector, as described above, the size of the light-receiving area (the sum of divisional light-receiving areas 1 to 4) required for performing proper tracking control is established so as to be larger than the size of the actual light-receiving area A to which a bundle of luminous flux is applied stationarily.
In such a light detector, if separating areas 5 are made wider, the area in which received light cannot be detected becomes larger so that it becomes impossible to perform accurate detection and communication of information. It is therefore necessary to make the separating areas 5 as narrow as possible.
Since a conventional light detector is so configured as described above, if separating areas 5 are made narrow, signals interfere with each other because of coupling between adjacent ones of the divisional light-receiving areas 1 to 4, thereby producing so-called crosstalk which influences the accuracy of the light detector, particularly in the case of a signal of a high frequency.
In such a heterodyne system or the like, therefore, there has been a problem in that when a tracking control signal or the like is obtained through phase-comparison of the respective outputs of the respective divisional light-receiving areas 1 to 4 in a high band, it is necessary to selectively determine whether the width of separating areas 5 is to be made wider in order to prevent crosstalk at the sacrifice of accurate detection and communication of information, whether the width of separating areas 5 is to be made narrow at the sacrifice of frequency characteristic, or whether the light-receiving area of the light detector is to be made smaller at the sacrifice of ease of initial adjustment.
That is, generally, a light detector is moved in order to adjust and put the light detector in a predetermined position, and the position in which the light detector is put in the beginning of the adjustment is generally selected in a broad range which is a 100 times as wide as the actual light-receiving area A. In actual adjustment, a user determines the position while monitoring the output of the light detector. Since a light detector is small in size and the light-receiving area thereof is narrow, the work necessary to search a bundle of luminous flux (a light spot) on the light detector has been very troublesome.
The reason why the position in which the light detector is to be put in the beginning of adjustment is so wide is that the position of the light detector is displaced when it is housed in a package, or the adjustment-initiating position of a position-adjustment mechanism is quite uncertain.
Taking the labor of adjustment into consideration, therefore, it is easy to adjust the light detector if a light detector is provided at the initial position so that the quantity and direction of displacement of the light detector can be determined; but if there is no light detector in the initial position, it is impossible to determine the direction in which the position of the light detector is to be moved, so that much time and labor are required for the search of the position to which a bundle of luminous flux is applied, and it is difficult therefore to automatically adjust the position of the light detector.
However, if the light-receiving area of the light detector is enlarged, the coupling capacity between adjacent ones of the divisional light-receiving areas 1-4 becomes large. For example, in a case, as has been known as a heterodyne system or a time difference system, in which a tracking error signal or the like is obtained on the basis of the quantity of phase shift between the respective high frequency signals of adjacent ones of the divisional light-receiving areas 1 to 4; that is, in a case in which a phase difference signal is required over a high frequency band, it becomes impossible to obtain this phase difference signal if crosstalk is produced in the high frequency band by the coupling capacity.
As shown in FIG. 9, therefore, the phase difference transfer characteristic of the light detector illustrated by curve II is deteriorated to a greater extent than the frequency of the phase difference signal illustrated by curve I, and there has been a problem in that it is difficult to obtain a light detector which is easily adjusted and which has little crosstalk, in the recent circumstances where an optical disk unit of high density and broad band is required.
Curve III in FIG. 9 shows the transfer characteristic of a phase difference signal in the case in which the separating areas 5 are made narrow.
According to the heterodyne system, a tracking control signal is obtained through a circuit shown in FIG. 10 on the basis of the respective outputs of divisional light-receiving areas 1 to 4 of the light detector shown in FIG. 6.
In FIG. 10, reference numeral 21 represents an adder for adding the respective outputs of divisional light-receiving areas 1 and 3; 22 represents an adder for adding the respective outputs of divisional light-receiving areas 2 and 4; 23 represents an adder for adding the respective outputs of adders 21 and 22; 24 represents a subtracter for subtracting the output of adder 22 from the output of adder 21; 25 represents a rising pulse-generating circuit (RPG) for supplying a sampling pulse on the basis of the output of adder 23; 26 represents a falling pulse-generating circuit (FPG) for supplying a sampling pulse on the basis of the output of adder 23; 27 and 28, respectively, represent gates for passing the output of subtracter 24 on the basis of the respective outputs of RPG 25 and FPG 26; 29 and 30, respectively, represent holding circuits for holding the outputs of gates 27 and 28; and 31 represents a subtracter for subtracting the output of holding circuit 30 from the output of holding circuit 29, the output of subtracter 31 being used as the tracking error signal TE.